Circumstellar Disks

A good place to start when learning about circumstellar disks is
understanding why we study them. The most
important and exciting reason we study circumstellar disks is
because they are believed to be the sight of planetary formation.
Furthermore, by studying these disks, we are able to constrain
theories of planetary formation and can develop timescales for the
evolution of planetary development. Finally, addressing these
issues allows us to compare our own solar system to others, placing
our own solar system within a context of other systems.

It is now believed that our own sun, like all other stars, formed from
a cloud of dust and gas in space. For some reason or another a local
density increase occurs within these clouds. This causes that portion of
the cloud to contract in on itself under the influence of its own
gravitational pull. In the end, the cloud collapses in on itself to
create what is known as a protostar. A protostar is a star that does
not derive its energy from fusion; there is not enough
gravitational force yet to cause fusion within the star's core. It
still glows, however, because the gravitational contraction causes the
gas to become so hot that it glows at optical and infrared
wavelengths.

Though the star has collapsed in on itself, dust still remains. What
is left
of the cloud rotates with the protostar and begins to flatten into a
disk. A significant portion of the left over dust and gas spirals into
the protostar adding to its mass. This is known as accretion. There is
still remaining material, however, that has enough angular momentum to
stay in orbit around the protostar. The disk continues to flatten and
accretion stops. The disk's thickness becomes very small compared to
its radius, astronomers call this type of disk geometrically thin.

The disk is very dense. The grains are subject to many forces and
collide with each other often. Some grains begin to stick
together. Soon large asteroids form and start to attract other pebbles
gravitationally. These bodies quickly grow to the size of small
planets. The terrestrial planets in our own solar system are large
accumulations of these bodies. Other planets also form further from
the central star. It is cooler in this region of the disk which
allows some of the larger rocky cores to accrete gas. These cores will
become huge gaseous planets like the gas giants found in our solar
system.

The picture of the newly forming planetary system heavily resembles
that of our own. Unfortunately, it is the only one we can observe in
detail, so our view of planetary formation is heavily biased. There is
still a lot we don't know. For instance, are planetary systems like
ours common? Do they have to have structures similar to our own, or is
our solar system unusual? Might there be life on some of these planets?

So the question now is how do we study circumstellar disks and the
planets that might be forming within them. In an ideal world, we could
simply point our optical telescopes at a star with a circumstellar
disk or planetary system and study its geometry. All we would
have to do is take a picture and look for the planets. Unfortunately
it is not that simple. The light of the star overpowers any energy
emission a planet could produce. Furthermore, determining physical
characteristics from an optical image of a circumstellar disk has many
limitations.

Just take a look at the image of IRAS 04302+2247 at the top
of this page. What you are looking at is a nebula that is cut in
half by the dust of a circumstellar disk. The nebula itself is
illuminated by the star but the dust of the disk, which is almost edge
on to our field of view, absorbs all of the optical light from the star
hiding it from our view. It is for this reason that infrared radiation
is so useful for studying these objects. Although, the disk is
completely dark in the optical range, it glows brightly in the
infrared.

So, how might one extract more information from this disk? As was
stated before, the disk itself emits heavily in the infrared. We can
therefore use this to our advantage and develop ways of looking at the
data which might allow us to determine some of its physical
characteristics. We might also be able to determine where
planets are forming, not by detecting the planets themselves but by
using "tracers". By "tracer" we mean finding more easily detectable
characteristics that planets produce, and using them to diagnose
possible planetary structures.